One such cause is that shales and sands that are being buried deeper because of additional deposition on top must compact to stay at normal pressure.
These shales and sands can only compact, however, if the associated water is allowed to leak off. If this water cannot bleed off, the formations will exhibit geopressures i.
vcbackyard.com/4548.php Drilling wells in any formation pressure environment requires the weight of the drilling mud to be balanced against the pressure of the formation being drilled. The fastest and most efficient drilling rates are obtained when an overbalance of mud to formation pressure is held to a minimum. The penetration rate begins to decrease dramatically when overbalances exceed about psi more than formation pressures at 10, to 12, feet. This is only about 0.
Further it is dangerous to drill with mud weight pressures that exceed formation pressures by about psi which is about 2. When mud is lost in one zone, the entire mud column drops decreasing the hydrostatic mud head and overbalance across other zones and even probably getting into an underbalanced condition across these other zones. When this happens, the differential pressure of higher formation pressure than mud pressure will allow flow of formation fluid into the well bore. This can literally cause the entire mud column to be blown out of the hole resulting in a catastrophic blowout and loss of the hole, drilling rig, and endangering the lives of the rig personnel.
Also when m'ud weight pressure to formation pressure is excessive as when overbalance exceeds about l'psi, there is a tendency for the drill pipe to stick due to this differential pressure.
To get unstuck sometimes can be very expensive or even impossible with presenttechnology; thus the well has to be abandoned with great financial loss. The knowledge of impending abnormal pressures enables the drilling engineer to prepare and perform the drilling in a safe and efficient engineering manner, since he is aware of the impending difficulties and problems. Present methods used in pressure detection such as wire line logs, i. Further, wire line logs must be considered as after-the-fact since they have the inherent drawback that the abnormal pressures can only be detected after the zone has been drilled.
In many instances, getting pressure information at this time is too late as drilling problems such as pipe sticking and well blowouts occur when the abnormal pressure zones are being penetrated.
Other methods of abnormal pressure detection while drilling include bulk density measurements of the drilled shale cuttings, drill penetration rate, torque or drag on the drill pipe, mud pump pressure, mud pit level changes, measurement of gas in mud system and clay mineral changes.
These methods for pressure detection are generally faster than the wire line logging techniques, but they all have the same drawback in that none of these guarantee the ahead-of-bit prediction in all cases. The drilling industry is in need of a method for predicting and detecting abnormal pressure zones prior to drilling into them. It is an object of this invention to provide a method of predicting and detecting pressure changes before drilling them. It is another object to drill geopressured formations without danger of a blowout.
It is also an object of this invention to keep mud weights at a safe minimum during drilling so that loss of circulation does not occur. It is a further object to drill abnormal pressured formations at a high penetration rate without ceasing drilling operations to detect such abnormal pressures.
Other objects, advantages and features of this invention, will become obvious from the following specification and appended claims. The normally pressured hydrostatic pressured portions of the strata are drilled according to well known techniques in which a drilling fluid is circulated in the borehole.
Purchase Origin and Prediction of Abnormal Formation Pressures, Volume 50 - 1st Edition. Print Book & E-Book. ISBN , of early formation analysis of abnormally pressured zones was primarily to predict Abnormal Formation Pressures, Elsevier, Amsterdam () Gurevich A.E., Chilingarian chupopernoro.gq on the origin of formation fluid pressure: well-logging.
While drilling the normally pressured rock, the drilling fluid is maintained at a relatively low weight, i. During this drilling operation the concentration of the sulfate or carbonate ions contained in the formation is system-actically and periodically or continuously determined.
This is done by measuring at the surface the activity or concentration of either the sulfate ion or the carbonate ion in the drilling mud, cores or shale drill cuttings removed from the well at various depth intervals. In the normally pressured formations the concentration of the sulfate and carbonateions changes only slightly with increasing depth.
However, several hundred feet above a geopressured interval the concentration of these ions begins to either increase or decrease rapidly. When this change in activity occurs, it is a signal that a geopressured zone lines somewhat below the drill bit in yet undrilled rock strata. Thus, this early warning of impending geopressure permits the drilling engineers to start controlled drilling procedures. These procedures, such as keeping a constant rotary speed and weight on the bit while monitoring penetration rate, will alert the driller when the geopressure is reached since the penetration rate will begin to increase under these controlled procedures at this time and the geopressures will not be masked by uncontrolled conditions.
The weight of drilling fluid can then be adjusted to compensate for the change in formation pressure. Drilling a well in the above described method provides the fastest and most efficient drilling, but most important permits the safest drilling. Controlled drilling procedures require special precautions which makes their use throughout the entire drilling operation technically difficult and uneconomical.
Where a drilling mud is used, the pressure of the column of drilling mud against the formation is increased by increasing the density of the drilling mud as by adding to the mud barium sulfate or some other weighting agent. If air or mist drilling is being employed, the pressure is increased by increasing the amount of air being compressed. Formation fluids contain a wide variety of water soluble ions. Among such ions are the sulfate ion and the carbonate ions. In normally pressured formations the concentration or activity of the sulfate and carbonate ions is relatively constant with increasing depth.
The reason for this phenomenonis not known with certainty. It has been postulated that water is squeezed out of shales as they compact due to the weight of overlying sediments. However, the shales act as a filtration membrane and the water that is squeezed out is fresh leaving the ions behind in the shale and therefore increasing the shale ion concentration or activity.
Since overpressured shales are undercompacted for their depths they still contain considerably more water and the soluble ion concentration or activity is less than in highly compacted shales. When compacted shales with the high ion concentration or activity overlie the undercompacted shales with low soluble ion concentration, or activity, the shales tend to try to come to ionic equilibrium by ion diffusion from the concentrated solution to the dilute solution normal pressure interval to the abnormal pressure interval in opposition to the membrane filtration effect.
This in theory creates the interval of to feet above the geopressure zone where diffusion of ions has caused a freshening of formations waters. This invention therefore has developed a procedure to detect this zone of freshening waters above the abnormal pressure. Clastics strata encountered in the wells drilled in the Offshore Indus Basin of Pakistan Indus Marine-1A, Indus Marine-1C are characterized by the development of abnormal formation pressures.
The gradient and pressure cross-plots generated for these wells show the relationship of pressure with increasing depth. The study concludes that rapid burial and high rates of sedimentation during the Miocene Epoch Unable to display preview. Download preview PDF. Skip to main content. Advertisement Hide.
This is a preview of subscription content, log in to check access. Khan, N. Holm, G. Oil Gas J.
Shah, Z. Peshawar 30 , 13—40 Google Scholar. Raza, H. Fertl, W. Origin of abnormal formation pressures. World Oil 7 , 67—71 Google Scholar. Hubbert, M. Mechanics of fluid filled porous solids and its applications to overthrust faulting. Law, B. SPE disclaims any and all liability for your use of such content. More information. Abnormal pressure overpressure conditions in the subsurface can pose significant drilling hazards if not detected. This article discusses how acoustic logs can help to identify overpressure situations.
Abnormal pressure is defined as any departure from normal hydrostatic pressure at a given depth. Early and reliable detection of geopressure is vital to avoid or mitigate potential drilling and safety hazards, e. During drilling, advanced warning of approaching geopressuring enables the mud weight to be adjusted to avoid well and reservoir damage and to determine casing points.
This is a particular concern in deepwater wells in which the pressure difference; i. Geopressuring in hydrocarbon reservoirs may result from a variety of geologic and tectonic processes.
These methods are used in              :. Undercompaction is the primary mechanism for creating overpressure, particularly in deltaic basins in which high rates of deposition commonly prevent the escape of pore water trapped in shales.